During cataract formation, the soluble and membrane proteins of the lens are altered. The incubation of whole rat or mouse lenses in vitro has been useful in studying the many steps in cataract formation. Alterations observed with the in vitro incubation of lenses are similar to the changes in lens proteins during cataractogenesis. Many of these alterations can be demonstrated if homogenates of lenses are incubated at 37 degrees C for two days. These changes, which mimic the post-translation modification of lens proteins in vivo, are very specific and involve alpha and some beta crystallin subunits. Results of experiments on the lens homogenates suggest the post-translational modifications may be the result of proteolytic degradation. These changes are potentiated by calcium and would be consistent with the action of a neutral proteinase activity. Studies of this calcium activited protease and correlation of this activity with cataract formation can now be performed using the lens homogenate system.